Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling

An Oracle White Paper
June 2009
Using Relationship Diagrams to Enhance the
Critical Path Method of Project Scheduling

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Oracle White Paper—Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling
Executive Overview
This white paper provides an overview of the Critical Path Method (CPM) for project
scheduling and the history of its development. The implementation of the Relationship
Diagramming Method (RDM) variant of CPM within Oracle’s Primavera Risk Analysis
(Release 8.2 or later) is illustrated, and the benefits of such an approach are highlighted.
Primavera Risk Analysis is certified to be compliant with RDCPM, and it allows the user
preparing a CPM to better model the real world. In addition, the user reviewing the CPM
submitted by another can perform a more in-depth evaluation.
Introduction
The CPM for project planning is based on a mathematical algorithm for scheduling
activities. It is an important tool for effectively managing projects ranging from
construction to software development to engineering. To model a project using CPM, the
following inputs are required:
 A list of all activities required to complete the project
 The duration of each activity
 The dependencies between activities

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Oracle White Paper—Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling
History of Project Scheduling Techniques
The original format for CPM, which was developed 50 years ago, was later renamed Arrow
Diagramming Method (ADM). ADM is also known as Activity on Arrow, and both of these
variants of CPM are distinct from the Precedence Diagramming Method (PDM) variant—also
known as Activity-on-Node—which was first implemented in 1964. Another variant of the same
era was the Program Evaluation and Review Technique (PERT), which the U.S. Navy developed
to create the Polaris missile system. PERT focused on clearly defined milestones rather than the
loosely defined activities between such milestones.
All three methodologiesADM, PDM, and PERTincluded weaknesses due to technology
limitations. The original ADM and PERT methodologies were designed to operate on
computers that lacked random access memory and were limited to linear access memory. Old
cartoons illustrating computers with large reel-to-reel magnetic tape highlight the limitations that
early software designers had to overcome. Similarly, PDM software was designed with fewer
features. It stored less information and required less memory than theorists had initially
envisioned. Many of these issues are rooted in this failure to record information known by the
planner or scheduler.
The Relationship Diagramming Method Variant of Critical Path
Method
In 2005, the construction industry manual CPM in Construction Management introduced RDCPM,
the RDM variant of CPM.1 RDCPM attempts to address the information-recording issues that
plagued ADM, PDM, and PERT. Figure 1 shows the trademark that attests to the certification of
proper use of RDCPM.



Figure 1. The trademark showing compliance to the RDM variant of CPM



1 James O’Brien and Fredric Plotnick, CPM in Construction Management, 6th ed. (McGraw-Hill, 2005).
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Oracle White Paper—Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling
Just as CPM and PERT were distinguished from bar charts and milestone charts by the recording
of additional information used to create those charts, RDM is also distinguished from the older
versions of CPM and PERT by recording even more of that information. This information, as
implemented by Primavera Risk Analysis, can be generally classified into several additional
code fields:
 Reason/why codes and description fields
 Events and additional types of restraints between activities
 Relationship codes to further indicate the relationship between two activities
Reason/Why Codes and Description Fields
Reason/why codes provide additional description about why a restraint or link has been placed
between two activities. This intelligent code can be entered based upon the thought process of
the project team or calculated from other information about the activities. The code can also
be used for additional calculations, such as better diagnostics for validation of plan logic, more-
robust resource-leveling algorithms, acceleration or crashing studies, and other what-if scenarios.
The reason/why code is supplemented by additional text fields that can contain descriptive
information.
The reason/why code is supported in Primavera Risk Analysis (Release 8.2 or later) by clicking
Plan > Links > Link Categories > User Fields. The default reason/why codes—or the link
category codes in the Primavera Risk Analysis implementation—are “0” undesignated, “1”
physical, “2” just-in-time, and “3” resource. Additional code designations, such as by individual
resource, can be assigned from 4 through 255.
In the example in Figure 2, users augment the link category codes by entering descriptions such
as “same crew” and “window wall affixed at edge of floor slabs.” In the four text fields, users can
enter additional descriptions or special coding assigned to the restraint or link between activities.





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Oracle White Paper—Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling
Figure 2. An example of link categories and user fields in Primavera Risk Analysis
Code Value 2: Just-in-Time
Code value 2 is reserved for a special type of link category: the just-in-time reason/why code.
This code represents a physical relationship between activities that depend on the predecessor
activity being completed just in time to support the early start of the successor activity. A typical
use is between the end of a string of procurement activities (submit, approve, fabricate, and
deliver) and a production activity (such as “rig/set the procured item to a foundation as soon as
possible after the foundation is poured and cured”). Another example is a subcontractor being
expected to deliver or install a just-in-time component to enable a continuous workflow for the
prime contractor. The calculation of the just-in-time dates for an activity is similar to the
calculation for the late dates. It starts at the last activity’s late finish and flows toward the start of
the project. The just-in-time finish and the start dates for each activity are the same as the late
finish and start dates. However, at the link set to link category 2, the just-in-time finish of the
predecessor activity to the link is set to the early start of its successor. Further processing
continues as previously noted. Users of this feature might choose to display the late start and
finish of the latest dates before causing delay to the project. Or they might choose to display the
just-in-time start and finish of the latest dates before causing a disruption to the production
crew’s progress.

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Oracle White Paper—Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling
Events and Restraints Between Activities
Another hallmark of RDM is the re-establishment of the concept of events, which is generally lost
in the transition from ADM and PERT to PDM. RDM has events at the beginning of an activity,
at the end of an activity, and embedded within an activity whenever some portion of an activity
must be completed before the start or finish of another activity. RDM also permits tags,
descriptions, and user-defined codes to be assigned to these events. Because RDM handles
events in this manner, users can record, for example, exactly what percentage of an activity is
required to be complete before another can start.
Primavera Risk Analysis implements this RDM requirement by permitting activities to be split,
placing the partial activity descriptions on the individual subtasks within the activity. RDM also
permits the user to distinguish between the following two scenarios:
 Activity B will start 3 days after Activity A’s 10-day process has begun—without regard to
whether any progress was made on days two and three.
 Activity B will start only when 3 days—or 30 percent—of Activity A’s 10-day process has been
completed.
This is accomplished in Primavera Risk Analysis by the split activity function. To implement the
first scenario, a traditional start-to-start restraint with a three-day delay might lead from the start
of Activity A to the start of Activity B. To implement the second scenario, the RDM progressed-
to-start restraint might lead from the end of a three-day subtask within Activity A—with zero
delay—to the start of Activity B. Both uses will result in the same initial bar chart, but they will
result in different actual bar charts if Activity A starts and stalls on day two.
Relationship Codes
Relationship codes allow users to program the computer to view the relationship between
activities connected by a restraint or link. Users can then perform various diagnostic or global
changes based upon such review.
For example, suppose a person wants to transfer a CPM instruction from a PDM to an RDM.
Under the PDM, the instruction has codes assigned to activities that indicate the resource used as
well as the location of the work being performed. As a result, rules can be written to set the link
category to “physical” (code value 1) for a transfer from one craft resource to another at the
same location, and to “resource” (code value 3) for a transfer from one location to another using
the same resource. Reports can be prepared highlighting a handoff from one subcontractor to
another that requires additional supervision by the general contractor, engineer, and owner. To
account for the tearing down and rebuilding of scaffolds, for example, the user might implement
a global change that sets a two-day lag for each finish-to-start restraint whenever the masonry
subcontractor changes location. The uses of this new feature are almost infinite.
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Oracle White Paper—Using Relationship Diagrams to Enhance the Critical Path Method of Project Scheduling
Conclusion
With RDM, planners can incorporate the following information into their project plans:
 Reason/why codes and description fields
 Events and restraints between activities
 Relationship codes that indicate the relationship between two activities
The evolution of CPM to incorporate and understand these important relationships has allowed
project planners and schedulers to achieve more-accurate project plans. Primavera Risk Analysis
is certified to be compliant with RDCPM, and it allows the user preparing a CPM to better
model the real world. In addition, the user reviewing the CPM submitted by another can perform
a more in-depth evaluation. Oracle’s Primavera Risk Analysis allows the project team to better
monitor the progress of a project and to make midcourse corrections when needed.



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Using Relationship Diagrams to Enhance the
Critical Path Method of Project Scheduling

June 2009

Author: Fredric L. Plotnick, PhD, Esq., PE
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